FIG. 1A schematically shows an exemplary thyristor-based protection circuit. The thyristor is illustrated as its equivalent circuit including two bipolar transistors QP and QN. The transistor QP is a PNP transistor whose emitter forms the anode A of the thyristor, and the transistor QN is an NPN transistor whose emitter forms the cathode C of the thyristor. The base of the transistor QP is connected to the collector of transistor QN and forms the anode gate GA of the thyristor. The base of the transistor QN is connected to the collector of transistor QP and forms the cathode gate GC of the thyristor.
In the illustrated protection circuit, the cathode C is connected to the ground Vss, and the anode A is used to protect an input/output terminal of an integrated circuit, or a specific component of the circuit. The cathode gate GC is connected to ground via a depolarization resistor Rd. The anode gate GA is connected to ground via a voltage source Vt defining the latching threshold of the thyristor. The voltage source Vt is generally in the form of a diode stack or a Zener diode. A series resistor Rga represents the resistance of the gate GA.
FIG. 1B shows a current-voltage characteristic of a thyristor protection component of the type of FIG. 1A. The values I and V correspond to the anode current and the anode voltage. As long as the voltage V does not reach a threshold VBO defined by voltage source Vt and resistance Rga, the thyristor is blocked, which is indicated by the low level of the current I at the lower portion of the characteristic.
If the voltage V has a surge exceeding the threshold VBO, the thyristor turns on and pulls the voltage on anode A to ground, causing a sudden drop of voltage V and an increase of the current I. If the source of the surge has enough power, the current I grows rapidly, but the voltage V stays within acceptable limits.
Once latched, the thyristor remains on as long as the current remains above a hold threshold IH. The thyristor is actually latched when the gate current, here the emitter-base current of the transistor QP, exceeds a threshold noted IL on the characteristic. The threshold voltage VBO depends on the configuration of the control circuit of the gate. In some configurations the voltage source Vt is omitted, i.e. the gate resistor Rga is connected to ground Vss, and the latching threshold of the thyristor is set using only the value of the resistor Rga, so that:VBO=Vb+Rga·IL where Vb is the emitter-base conduction threshold of transistor QP.
The hold current IH of the thyristor is a parameter usually controlled by design. In some situations, it is also desirable to control the value of gate resistor Rga. Bipolar and CMOS technologies permit the manufacture of thyristors and the adjustment of their parameters using manufacturing steps that fit well within standardized manufacturing processes.
It may also be desirable to protect electro-optical components made of semiconductor materials against ESD and voltage surges. Optical and electro-optical components that one desires to integrate together are simple compared to components that are integrated in CMOS technology, whereby dedicated manufacturing techniques have been developed for optical components that are inexpensive compared to CMOS technologies. The techniques dedicated to optical components offer little flexibility for realizing protection devices such as thyristors.